Introduction: Stair gait is an activity performed daily. Inherently falls during stair gait continue to be a concern especially for older adults 65 years +. Recently falls have become the most common cause of injury-related deaths in individuals over the age of 75 y.o. Stair descent falls account for 75% of stair falls and also present a greater injury severity. Poor shoes or insoles and lighting condition can contribute to an increased risk of falls during stair locomotion. Stability can be measured using the COM-BOS ‘stability margin’ relationship. Center of pressure (COP), another stability measure,can be calculated from a multi-axis force-plate system. As well, plantar pressure is an important indicator of gait pattern efficiency. Aim: To identify aspects of stair gait that increase the risk of falls. By measuring the COM-BOS ‘stability margin’, the COP and plantar pressure patterns of individuals during stair gait, while modifying insoles and lighting. Methods: Young and older adults will ascend and descend a 4 level staircase, with two imbedded AMTI-force platforms in varying lighting condition (low, normal). Participants will be fitted with standardized footwear with Medi-logic insoles placed under varying hardnesses of insoles. An Optotrak motion capture system will record 12 IRED markers placed on the individual to determine the COM trajectory and BOS of location. Hypothesis: Partipants should demonstrate a greater lateral displacement in the single support phase during dim lighting as opposed to normal lighting. The stability of older adults will be compromised with alteration to the insoles (soft and hard).

Even prior to walking slower, older adults walk with a diminished push-off – decreased propulsive forces (FP) accompanied by reduced ankle moment and power generation. The purpose of this study was to identify age-related differences in the joint-level modifications used to modulate FP generation during walking. We posit that there are two possibilities for older adults to enhance FP generation. First, older adults may increase ankle power generation and thereby alleviate compensatory demands at the hip. Alternatively, older adults may opt to exacerbate their distal to proximal redistribution by relying even more on the hip musculature.
10 healthy young adults and 16 healthy older adults participated in this study. Subjects walked at their preferred speed while watching a video monitor displaying their instantaneous FP while instructed to modify their FP to match target values representing normal and ±10% and ±20% of normal. For all trials, we estimated lower extremity joint kinematics and kinetics.
During normal walking, older adults exerted smaller FP and ankle power than young adults. Enhancing FP via biofeedback alleviated mechanical power demands at the hip, without changes in ankle power. Further, older adults walked with increased FP without increasing their total positive joint work. Thus, given the same total requisite power generation, older adults got ‘more bang for their ankle power buck’ using biofeedback.Listed In: Biomechanical Engineering, Biomechanics, Gait

Falls due to slippery conditions are among the primary causes of disabling workplace injuries. Despite the extensive amount of human slip studies in the literature, only a handful of studies have reported ground reaction forces at the instant of slip initiation. The purpose of this study was to quantify the vertical ground reaction forces (VGRF) at slip initiation during unexpected human slips across different footwear-contaminant conditions. Forty-seven healthy subjects were unexpectedly exposed to a liquid–contaminant, while the vertical force was measured at the moment that the foot began to start slipping. The average VGRF were between 100 and 300 N and varied significantly across the footwear. These forces were significantly less than the typical forces (400-700 N) applied during slip-resistance measurements. This finding may suggest that available coefficient of friction (ACOF) measurements should use lower force levels in order to achieve higher relevance to the onset of slipping.Listed In: Biomechanics, Gait

Bilateral vestibular hypofunction (BVH) is a bilateral reduction or loss of vestibular function resulting in balance deficits and an increased falls risk. As part of a larger study, this experiment aimed to assess how spatiotemporal gait characteristics and their variability change across different walking speeds in patients with BVH. Nine patients (55±15y) with BVH have participated thus far. Experiments were conducted on the CAREN Extended system (Motekforce Link, Amsterdam, The Netherlands). Following multiple familiarisation trials, the participants completed five recorded two minute walking bouts at different speeds (0.6m/s, 0.8m/s, 1.0m/s, 1.2m/s and 1.4m/s). 60 strides per speed were analysed and the means, standard deviations and coefficients of variation (CV) of stride length and time, step length and width, double support time and swing phase toe clearance were calculated. Stride length, step length and toe clearance all increased with increases in walking speed (P<0.001). Stride and double support time decreased with increased walking speed (P<0.0001). No walking speed effect was found for step width (P=0.25). Significant reductions in variability with increases in walking speed were found for stride length, stride time, step length, toe clearance (P<0.01) and double support time (P<0.05). A significant increase in step width variability was observed with increases in walking speed (P=0.0033). These preliminary data suggest that while anteroposterior gait characteristics may improve in terms or variability with increases in walking speed in these patients, mediolateral motions may become more variable, which may have implications for mediolateral stability and falls risk in patients with BVH. Listed In: Biomechanics, Gait, Neuroscience

Old versus young adults exhibit increased hip and decreased ankle joint mechanical output during level and incline walking. This distal-to-proximal redistribution of joint torques and powers is now a well-established age-related gait adaptation and has been termed biomechanical plasticity. The effect of physical capacity, which varies greatly in old adults, on this gait adaptation remains unclear. For example, high capacity old adults (i.e. those with fast walking speeds) might either retain a more youthful gait strategy or adopt larger magnitudes of plasticity in order to walk well. The purpose of this study was to quantify the relationships between physical capacity and biomechanical plasticity in old adults during level and incline walking. We conducted 3D gait analyses on 32 old adults (>70 yrs) as they walked over level ground and up a 10° incline at self-selected speeds. We used motion capture (Qualisys AB) and force platforms (AMTI) to collect kinematic and ground reaction force data, respectively. To measure physical capacity, we used the SF-36 Physical Component score and to define biomechanical plasticity we created ratios of hip extensor to ankle plantarflexor peak torques, angular impulses, peak positive powers, and work. We conducted correlation analyses between SF-36 PC scores and the biomechanical plasticity ratios. Positive relationships existed between SF-36 PC scores and all biomechanical plasticity ratios during level walking. Similar results were observed during incline walking, however only three of these four relationships reached statistical significance. Our results suggest that old adults of higher physical capacities exhibit larger magnitudes of biomechanical plasticity.Listed In: Biomechanics, Gait

Purpose: Knee pain is a chief symptom of knee pathology. Both acute and chronic knee pain result in altered joint loads during walking, which potentially result in mechanical and biological changes in knee articular cartilage. Due to confounding factors in clinical knee pain (effusion, muscle weakness, inflammation, structural changes), it is difficult to examine the independent effect of knee pain on walking mechanics. The purpose of this study is to examine whether unilateral experimentally induced knee pain influences bilateral loading patterns during walking in healthy individuals.
Methods: This study was a controlled laboratory, cross-over trial. Each of 30 able-bodied subjects (M = 20, F = 10; 23 ± 2.4 yrs, 71 ± 12.7 kg, 178 ± 8.2 cm) completed three experimental sessions: pain (5.0% NaCl infusion), sham (0.9% NaCl infusion), and control (no infusion) in a counterbalanced order, 2 days apart (a washout period). For the experimental sessions, hypertonic (5% NaCl) or isotonic (0.9% NaCl) saline was continuously infused into the right (involved limb) infrapatellar fat pad using a portable infusion pump, which produced a continuous saline flow of 0.154mL/min (total 2.16 mL) for 14 min for the pain or sham session, respectively. No infusion was administered to the control session. Subjects and investigators were blinded regarding the saline solution which was being infused. During each of three experimental sessions, subjects performed 30-sec gait trials at a self-selected speed at two time points (pre- and post-infusion). Ground reaction force (GRF) data were collected using an AMTI instrumented force-sensing tandem treadmill (1200 Hz). The first 4 successful gait cycles in each limb were used for data analysis. A functional data analysis approach (α = .05) was used to detect time (pre- and post-infusion) x limb (involved vs. uninvolved) interactions for the vertical, anterior-posterior, and medial-lateral GRF.
Results: Significant time x limb interactions were observed during the pain session (hypertonic saline; 5.0% NaCl; p < .05). Experimental knee pain resulted in up to (i) 0.05 N/kg less vertical GRF and 0.02 N/kg more vertical GRF during various stance phases, (ii) 0.01 N/kg less breaking GRF during loading response, and (iii) 0.007 N/kg less lateral GRF and 0.007 N/kg more lateral GRF during various stance phases in the involved limb.
Conclusions: Relative to the pre-infusion condition, subjects during the knee pain condition tended to walk with less vertical, posterior and lateral GRF in the involved limb (painful limb) across various portions of stance, which simultaneously increased loads in the uninvolved limb (non-painful limb). Our data suggest that compensatory loading patterns occur simultaneously for the involved and uninvolved limbs. This unloading pattern in the involved limb may be due to perception of knee pain, which can make subjects feel fear for damaging or provoking pain more during walking. Moreover, voluntary and/or involuntary quadriceps inhibition (e.g., neuromuscular activation and strength) due to experimentally induced knee pain may play a role in reducing the loads in the involved limb because the quadriceps support the center of body mass eccentrically from initial loading response to midstance to prevent collapse of the lower limbs. These asymmetrical loading patterns due to knee pain and associated with neural inhibition may be a risk factor for knee joint disease progression via changes in mechanical components.Listed In: Biomechanics, Gait

Purpose:
Anterior cruciate ligament (ACL) tear greatly increases the risk of knee osteoarthritis (OA), even when patients undergo ACL reconstruction surgery (ACLR). Changes to walking kinematics following ACLR have been suggested to play a role in this degenerative path to post-traumatic OA by shifting the location of repetitive joint contact loads that occur during walking to regions of cartilage not conditioned for altered loads. Recent work has shown that changes to the average knee center of rotation during walking (KCOR) between 2 and 4 years after ACLR are associated with long term changes in patient reported outcomes at 8 years. Changes to KCOR result in changes to contact patterns between the femur and the tibial plateau. However, it is unknown if changes to this kinematic measure are reflected by changes to cartilage as early as 2 years after surgery. Ultrashort TE-enhanced T2* (UTE-T2*) mapping has been shown to be sensitive to subsurface changes occurring in deep articular cartilage early after ACL injury and over 2 years after ACLR that were not detectable by standard morphological MRI. Thus, the purpose of this study was to test the hypothesis that side to side differences in KCOR correlate with side to side differences in UTE-T2* quantitative MRI (qMRI) in the central weight bearing regions of the medial and lateral tibial plateaus at 2 years following ACLR.
Methods:
Thirty-five human participants (18F, Age: 33.8±10.5 yrs, BMI: 24.1±3.3) with a history of unilateral ACL reconstruction (2.19±0.22 yrs post-surgery) and no other history of serious lower limb injury received bilateral examinations on a 3T MRI scanner. UTE-T2* maps were calculated via mono-exponential fitting on a series of T2*-weighted MR images acquired at eight TEs (32μs -16 ms, non-uniform echo spacing) using a radial out 3D cones acquisition. All subjects completed bilateral gait analysis. Medial-lateral (ML) and anterior-posterior (AP) coordinates of average KCOR during stance of walking were calculated for both knees. Side to side differences in KCOR were tested for correlations with side to side differences in mean full thickness UTE-T2* quantitative values in the central weight bearing regions of the medial and lateral tibial plateau using Pearson correlation coefficients.
Results:
There was a distribution in UTE-T2* values, with some subjects having higher UTE-T2* and some lower in the ACLR knee relative to the contralateral knee. A significant correlation (R=0.407, p=0.015, Figure 1A) was observed between UTE-T2* and the ML KCOR with a more lateral KCOR corresponding to higher values of UTE-T2* for the medial tibia. Similarly, for the lateral tibia, a lower UTE-T2* was correlated with a more posterior KCOR (R=0.363, p=0.032, Figure 1B). Significant correlations were not observed for UTE-T2* in the lateral tibia with the ML position of KCOR or for UTE-T2* in the medial tibia with the AP position of KCOR.
Conclusions:
The results of this study support the hypothesis that side to side differences in mean full thickness UTE-T2* qMRI correlate with side to side differences in knee kinematics at 2 years after ACLR. The finding that a more lateral KCOR in the ACLR knee correlates with UTE T2* values in the medial tibia that were higher than the contralateral side suggests that this kinematic change, which has been previously shown to result in more relative motion between the femur and tibia in the medial compartment, could be affecting subsurface matrix integrity, inducing changes detectable by UTE-T2* mapping. Additionally, the finding that a more posterior KCOR in the ACLR knee correlated with UTE-T2* values in the lateral tibia that were lower than the contralateral knee further suggests that the UTE-T2* metric may reflect early changes in cartilage health. When interpreted within the context of prior work showing that a posterior shift in KCOR from 2 to 4 years post-surgery correlated with improved clinical outcomes at 8 years, the observed lower UTE-T2* with a more posterior KCOR, which is reflective of improved quadriceps recruitment, suggests positive cartilage matrix properties. In spite of the limitations of this cross-sectional and exploratory study, and the difficulty accounting for changes in the contralateral knee, these results support future studies of the relationship between UTE-T2* and KCOR to provide new insight into predicting the risk for OA after ACLR. Listed In: Biomechanical Engineering, Biomechanics, Gait, Mechanical Engineering, Orthopedic Research, Sports Science

INTRODUCTION. Increased age is associated with changes in gait mechanics and decreased muscle function. As the knee extensors (KE) are prime movers in gait, altered KE function (strength, power, fatigability) could alter knee mechanics. This study aimed to determine whether a bout of exercise induces KE fatigue and changes in knee mechanics in two older groups with different physical activity levels: sedentary adults and runners.
METHODS. Adults aged 55-70 who were either runners (≥15 miles/wk) or sedentary (≤3x30 min exercise bouts/wk) completed gait and strength testing before and after a 30 minute treadmill walk (30MTW). Joint kinematics were calculated using the point cluster technique. Externally-referenced moments were calculated using inverse dynamics. KE power and isometric strength were assessed via isokinetic dynamometry. Changes in KE power and knee mechanics were calculated; within-group changes were examined using paired t-tests (p<0.1).
RESULTS. Sedentary adults displayed a drop in KE power at 6/8 contraction velocities vs. 2/8 in runners (poster Figure 2). Both groups showed an increase in knee flexion angle at heel strike and runners displayed decreased knee flexion moments post-30MTW (poster Figure 3).
CONCLUSIONS. Vigorous physical activity may allow older adults to maintain fatigue resistance. Sensitivity of knee mechanics to KE fatigue remains unclear as few changes were seen even in a fatigued group. Global, rather than discrete, measures of joint function may provide more sensitive measures of the response of gait mechanics to muscle fatigue and may allow for a more complete picture of the impact of muscle function on gait.
Listed In: Biomechanics, Gait

Physical testing of TKR systems to assess stability is an important aspect in screening candidate TKR designs which can be expensive and time consuming. Costs can be reduced by utilizing 3D printed plastic components. The objective is to compare the kinematics and intrinsic constraint of metal-on-plastic (M-P) and plastic-on-plastic (P-P) implants under physiologically relevant loading, with and without simulated ligament contributions, in order to elucidate the effects of material pairings. A cruciate retaining TKR implant was created by combining a 3D printed ABS plastic tibial component with the standard cobalt chrome femoral component, as well as a 3D printed ABS plastic replica femoral component. This results in both M-P and P-P articulations that were mounted to a VIVO 6-DOF joint motion simulator (AMTI, Watertown, MA), which was used for in vitro constraint testing using functional laxity tests. Anterior-posterior (AP) and internal-external (IE) constraint was measured based on resulting deviations from the normal path when superimposed AP and IE loads were applied. Ligaments were simulated as tension-only point-to-point springs using the soft tissue modelling capabilities of the VIVO. Different kinematics were observed between the M-P and P-P implants which could be the result of different initial implant positioning on the joint motion simulator or due to “stiction” of the P-P implant. The functional laxity of the implant system tested appears to be relatively insensitive to the material pairing and ligament presence. These relationships are complex and hard to predict, which underscores the importance of pre-clinical in vitro testing.Listed In: Biomechanical Engineering, Biomechanics, Gait, Mechanical Engineering, Orthopedic Research